[SECMECH] Continuing my rant about EAP vs GSS-API...

Pasi.Eronen@nokia.com Tue, 02 August 2005 10:07 UTC

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Thread-Topic: Continuing my rant about EAP vs GSS-API...
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Subject: [SECMECH] Continuing my rant about EAP vs GSS-API...
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(Continuing and clarifying my comment in the BoF today...)

GSS-API establishes a "security context" between a client and a
server.  Depending on the GSS-API mechanism, establishing the security
context may use the help of a "trusted third party", either off-line
(like CA) or on-line (Kerberos KDC).

EAP includes only the communication between the client and "trusted
third party" (EAP server), but it does not establish a security
context between the client and the service. But that's exactly what
people interested in using a security framework (like ISMS or 802.11)
would like to do! So just EAP alone is not really a framework you can
compare with GSS-API.

To make this comparison more accurate, we _have_ to consider the real
"EAP+AAA" framework, instead of claiming that they're totally
separate.

Of course, it's still possible to e.g. analyze an EAP mechanism
without considering AAA. The result of this analysis could be e.g.
that the mechanism has certain security properties.  But from the
point of view of applications, what really matters is the security
properties of the whole framework (or this EAP mechanism when used
within this framework).  These don't trivially follow from properties 
of the building blocks (just like AES being secure doesn't make all
EAP methods that use AES secure).

In particular, an application like ISMS is really concerned with 
"ok, I have this security context that I'll use for protecting my
application-specific communication. What properties does this context
have? Who is the other party I'm sharing this context (and keys
contained in it) with?"

An analysis of an EAP mechanism, like EAP-TLS, can answer the question
"who is the other party I'm sharing the EAP-SA with" (just look at the
TLS server certificate).  But EAP alone cannot answer the
application's real question "who is the other party I'm sharing this
context with".

Or in other words, GSS-API mechanisms (that do mutual authentication)
do mutual authentication (and set up a security context/association) 
between the client and the service (application endpoint). EAP 
mechanisms do mutual authentication between the client and the 
EAP server; and that doesn't automatically mean mutual authentication
between the client and the service/application endpoint.

This difference is IMHO not an insignificant minor detail, and 
needs to be taken into account when talking about "bridging"
EAP and GSS-API mechanisms, or designing mechanisms that can 
be used in both frameworks, etc.

I'm not saying this deficiency in the EAP+AAA framework is serious
problem in all cases. In some environments the authentication
requirements in different directions are quite different.

For instance, in 802.11 network access the client's/user's identity 
is usually much more important; for the access point/network side, it 
may be enough to verify that "this is someone the TTP (EAP server)
trusts". This is essentially what EAP+AAA framework today does. 
Something like ISMS might have a similar situation, authenticating 
the identity of the SNMP manager might be much more important than 
the SNMP manager...

Best regards,
Pasi

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